For machining metal work-pieces, by cutting, turning, milling, drilling and like, cutting tools are used. To ensure that chips are efficiently removed from the work-piece, whilst ensuring long working life of cutting tool, a cutting tool insert is required to be hard and tough.
Hardness however may be correlated with brittleness. Being both hard and tough, composite materials consisting of hard ceramic particles in a metal matrix are very popular choices for inserts. A number of such ceramic metal composites or cermets have been developed. The so called hard metals or cemented carbides, in particular, WC—Co, consisting of tungsten carbide grains in a cobalt matrix, are the materials of choice for fabrication of cutting tool inserts for many applications.
Inserts remove chips and shape the work-piece, but are, themselves, worn away in the process. The wear of cutting tool inserts takes place at their contact surfaces with the workpiece, and can generally be attributed to mechanical, chemical and thermal interaction with the workpiece.
The downtime of machine tools whilst replacing insert is generally an expensive operation. Much research is directed to improving the wear resistance of inserts by application of hard coatings. Hardness is a measure of resistance to plastic deformation, and there is a correlation between hardness and wear resistance. While coatings increase wear resistance, they are often susceptible to catastrophic failure modes such as peeling and the like.
Coatings may be formed on inserts by a range of coating technologies that are generally classified as PVD (physical vapor deposition) or CVD (chemical vapor deposition).
PVD gives very good properties. Coating is only line-of-sight. PVD coatings are characterized by compressive residual stresses from the deposition process. Because of the risk of coating failure by peeling as the coating thickness increases, PVD is generally limited to thin coatings.
CVD coatings are not to line-of-sight. Furthermore, deposition temperatures are typically rather higher than those of PVD technologies and this facilitates the development of a diffusion-induced interface between coating and substrate which allows good adhesion to be achieved. Indeed, good adhesion is one of the critical requirements for the coating of inserts.
Furthermore, there are some materials and material-substrate combinations that are only practical by one or other coating technique.
For more than 40 years, CVD (chemical vapor deposition) has been used for coating inserts, thereby improving their performance in machining. Coatings of TiN, TiC and TiC,N may be deposited onto appropriate substrates by reacting titanium tetrachloride with other gases, and removing the gaseous chlorides thus formed:TiCl4+N2+H2→TiN+Chlorides and other gases.TiCl4+CH4+H2→TiC+Chlorides and other gases.TiCl4+N2+CH4+H2→TiCN+Chlorides and other gases.
Al2O3 coatings may be produced in a similar manner:Al+HC1+H2→AlCl3+H2;AlCl3+H2+CO2+H2S→Al2O3+Chlorides and other gases,
where H2S serves as a catalyst; enhancing the deposition rate and thickness uniformity of the Al2O3 coating.
It will be appreciated that, over the years, other chemical vapor deposition routes have become available for deposition of TiN, TiC, TiCN and Al2O3, and the above routes using titanium chloride and aluminum chloride are given by way of non-limiting example, only.
Indeed, a wide variety of hard coatings, such as various carbides, nitrides, oxides, borides and mixtures thereof may be deposited by one or other of the various PVD and/or CVD techniques. In the machining of hard materials, such as cast iron, for example, high temperatures are generated. At such high temperatures, many coating materials, such as carbides and nitrides are reactive, and may interact with the work-piece and/or with the cooling fluids and air. Al2O3 (alumina) is both highly chemical resistant and very hard. Consequently, alumina is a popular coating material for increasing the life of inserts.
European Patent Number EP1245700 to Ruppi entitled “Enhanced Al2O3—Ti(C,N) Multi-Coating Deposited at Low Temperature” relates to a coated body of cemented carbide, cermet, ceramic and/or high speed steel used as a metal cutting tool and having a multi-layer of κ-Al2O3 and/or γ-Al2O3 optionally interspersed with layers of titanium carbonitride Ti(C,N) which can also be applied by MTCVD.
Other known multilayered coatings that comprise both Ti based layers and Al2O3 layers can be found in the following references:
JP11269650A2JP2000119855A2 JP2000107909A2 JP03122280A2JP2000096235A2JP2000096234A2JP11347806A2JP11310878A2JP2002144109A2JP2001062604A2JP2000158208A2JP2000158207A2EP0594875A1JP59025970A2JP2005246596A2JP2004188577A2U.S. Pat. No. 6,689,450 JP2004299023A2JP2004299021A2JP2004188576A2JP2004188575A2JP2000052130A2JP11254208A2JP11090737A2JP11000804A2JP10310878A2JP10310877A2
Alumina exists in a number of forms. γ-alumina and κ-alumina are metastable phases. In a number of machining scenarios, such as interrupted turning on a lathe, and when turning without a coolant, for example, inserts coated with α-alumina has been found to perform better than those coated with γ-alumina or κ-alumina.
From the alumina phase diagram, at temperatures of above 1050° C. under standard pressure, γ-alumina and κ-alumina transpose into the stable α-alumina allotrope (corundum). Due to the poor heat dispersing properties of alumina and the high temperatures generated in machining, it is likely that the rake surface reaches 1050° C. in some cutting processes, particularly with dry machining, and, at the very high pressures experienced on the rake face of inserts, the phase transformation is likely to occur at lower temperatures. The recrystallization of the alumina surface of an insert during machining processes may lead to accelerated wear.
It is also possible that at the high temperature-pressure conditions experienced by inserts during cutting processes, where temperatures of 1000° C. are not uncommon, and various mechanisms such as slip, twinning, grain boundary sliding and possibly diffusional creep come into play in α-Al2O3 and this allows sufficient ductility to prevent brittle failure. See “Nanoindentation Hardness, Microstructure and Wear Resistance of CVD α-Alumina and κ-Alumina Coatings” by Ruppi, Laarsson and Flink.
The growth process, the structure and properties of the deposited layer are governed by deposition conditions such as the nature and temperature of the substrate and the contents and kinetic energies of the gas flux. The rhombohedral α-alumina phase which is the stable allotrope and the hardest of the various polymorphs is generally achieved at deposition temperatures of 1000-1100° C. See for example, Prengel et al. Surface Coatings Technology, 68-69, 217 (1994), although deposition at 580° C. has been achieved by plasma assisted CVD. See Krylov et al. for example, Appl. Phys. A 80 1657 (2004). By use of an appropriate template (i.e. favorable substrate), specifically Chromia, Andersson achieved α-alumina deposition at temperatures of as low as 280° C. by reactive magnetron sputtering. See Ph.D. thesis, “Controlling the Formation and Stability of Alumina Phases” by Jon Martin Andersson, Linköping Universitet, Institute of Technology, Linköping 2005.
Relatively thick coatings of α-alumina may be deposited using CVD techniques. However, such coatings generally display directed grain growth through the coating thickness, resulting in a columnar microstructure that is somewhat susceptible to crack propagation.
A known technique for avoiding columnar growth in κ-alumina is to periodically interrupt the deposition of the κ-alumina by depositing a thin layer of a different material, after which deposition of κ-alumina may be resumed. TiN is one such material that has been used to interrupt the growth of κ-alumina. See “Microstructure and Deposition Characteristics of κ-Al2O3” S. Ruppi & A. Larsson Journal de Physique IV France 8 (1999), EuroCVD 12, Part 8 350-355. By periodically introducing a thin layer (0.1 μm to 1 μm) of TiN, the κ-alumina crystal growth can be arrested and a new layer of alumina can be nucleated. The resultant κ-alumina is almost equiaxed and displays significantly better resistance to crack propagation than the columnar microstructure typical of CVD κ-alumina.
Unfortunately, the technique does not provide good results for cutting inserts and the like, since the κ-alumina-TiN layers have relatively poor adhesion, resulting in coating peeling, which may actually result in accelerated wear.
There is thus still a need for thick, high quality, α-alumina coatings that show high ductility and a low tendency to both crack propagation and peeling, and the present invention addresses this need.